A filter circuit is used as a RF circuit of mobile devices such as cellular phones. The filter circuit passes RF signals in a passband and attenuates signals out of the passband. The filter circuit may be configured to increase the attenuation in a particular frequency band outside of the pass band. Japanese Patent Application Publication No. 2009-33733 and PCT Pamphlet No 2009/025106 disclose methods of increasing the attenuation in the particular frequency band outside of the passband by connecting a phase shifter in series to the filter.
However, the use of the phase shifter connected in series to the filter increases the insertion loss.